RINGS OF THE GAS GIANTS …all 4 gas giants have ring systems… Saturn Jupiter Neptune Uranus

Jupiter –broad, dark, fine particles (smoke-sized) Saturn – broad, bright, complex, icy particles (few cm to several meters) – reflect 70% sunlight……short-lived? (millions of years?) Uranus – narrow, dark, fine particles Neptune – uneven, dark fine particles All consist of independently orbiting small chunks of silicate, carbonate and water ice material within very thin layers. … Saturn’s rings span 360,000 km, but are only a few metres thick in places.

Origin? Tidal forces destroy a large solid moon insides a planet’s Roche limit. Major ring systems are always found inside the Roche limit (about 1.44 planet diameters above the surface). Collisions make rings the final configuration for swarms of individual particles in orbit; inter-particle collisions efficiently dissipate any net ang. mv. in the perpendicular direction  thin disk

Note: Roche limit really does seem a good marker for ring edges Some satellites ARE found inwards of the Roche limit…? All the rings have complex structures (gaps) Ring behaviour at least partly controlled by satellites:

Orbital Resonance Orbital period of O2 is integral multiple of the orbital period of O1 The stronger gravitation pull on O1 in configuration 1 causes it to move into an elliptical orbit. Configuration 3 O1 has completed one rotation, but O2 has just finished half of its rotation O2 Configuration 1 O1 O1 Configuration 2 O2

Lagrange Points The Italian-French mathematician Lagrange  5 special points in the vicinity of two orbiting masses where a third, smaller mass can orbit at a fixed distance from the larger masses. The Lagrange Points mark positions where the gravitational pull of two large masses equals the centripetal force required to rotate with them.

Lagrange Points 5 Lagrange points  represent points in space where an unmoving particle would remain very stable. Two of these points (L4 and L5) form equilateral triangles with the primary masses. The other three lpoints (L1, L2, L3) are colinear with the primary masses…

Cassini  Temperature and Composition Vis/UV spectra indicate rings are predominantly water ice (could be other ices e.g. methane, but not yet detected) Some rings show reddening, due to contamination (e.g. dust) or radiation effects Cassini colour-coded UV image; blue indicates more water ice present. Note the sharp compositional variations

Cassini  Grain Size

Ring Lifetimes Small grains (micron-size) have lifetimes of ~1 Myr due to drag from plasma and radiated energy So something must be continuously re-supplying ring material: Impacts (on satellites) and mutual collisions may generate some Volcanic activity may also contribute (Io, Enceladus) New evidence for Enceladus sculpting the E-ring!.......also evidence for Enceladus being active.

Complex structure of Saturn’s rings: Coarse structure – radial variation in optical depth  due to differences in particle sizes (e.g. due to collisions and proximity to planet). Fine structure – gaps between rings caused by resonance interaction with satellites  `shepherding moons’: gravitational acceleration or deceleration of dust particles  moons clear paths around themselves and create very narrow rings.

Braiding – also caused by resonance interactions with moons – it not an actual inter-twining, but more a departure from purely circular paths. Radial spokes – electrical and/or magnetic effects  temporary clumping of particles usually last ~ few hours (< Prot)

Why the sharp edges? Keplerian shear blurs the rings Particles closer in are going faster Collisions will tend to smear particles out with time – this will destroy sharp edges and compositional distinctions collision faster slower Shepherding satellites Outer satellite is going slower than particles Gravitational attraction subtracts energy from particles, so they move inwards; reverse true for inner sat. So rings keep sharp edges And gaps are cleared around satellites

Jupiter’s rings

Jupiter’s rings The ring is now known to be composed of three major components. The main ring is about 7000 km wide and has an abrupt outer boundary 128,940 km from the center of the planet. The main ring encompasses the orbits of two small moons, Adrastea and Metis, which may act as the source for the dust that makes up most of the ring. At its inner edge the main ring merges gradually into the halo. The halo is a broad, faint torus of material about 10,000 km thick and extending halfway from the main ring down to the planet's cloudtops. Just outside the main ring is a pair of broad and exceedingly faint gossamer rings, one bounded by the orbit of the moon Amalthea and the other by the orbit of Thebe.

Jupiter’s rings Even the Main Ring is very faint (~1.7 – 1.8 RJupiter) All rings consist of very small particles The Halo is inside the main ring (~1.3 RJupiter)…ionized dust particles elevated by E- , B-forces The two Gossamer rings are outside the Main ring All the inner satellites are connected to the ring structures

Jupiter’s main ring, from Galileo

A faint mist of particles can be seen above and below the main rings; Jupiter’s Ring Halo A faint mist of particles can be seen above and below the main rings; this vertically extended, toroidal "halo" is unusual in planetary rings, and is probably caused by electromagnetic forces which can push small grains out of the ring plane. In order to accentuate faint features in the image, different brightnesses are shown through color, with the brightest being white or yellow and the faintest purple.

Jupiter’s Gossamer Ring Jupiter’s gossamer ring, from Galileo

Metis (diam. 40 km) is embedded in the main ring Adrastea (diam. 20 km) is at the main ring’s outer edge Amalthea (diam. 190 km) is at the outer periphery of the inner Gossamer ring Thebe (diam. 100 km) is near the outer periphery of the outer Gossamer ring

Uranus - system with rings This series of images from Hubble Space Telescope shows how the ring system around the distant planet Uranus appears at ever more oblique (shallower) tilts as viewed from Earth - culminating in the rings being seen edge-on in three observing opportunities in 2007.

Uranus - system with rings Note the rotation axis and position of rings

Uranus - system with rings The infrared image allows to probe the structure of Uranus' atmosphere, which consists of mostly hydrogen with traces of methane. The red around the planet's edge represents a very thin haze at a high altitude. The haze is so thin that it can only be seen by looking at the edges of the disk, and is similar to looking at the edge of a soap bubble. The yellow near the bottom of Uranus is another hazy layer. The deepest layer, the blue near the top of Uranus, shows a clearer atmosphere. Image processing has been used to brighten the rings around Uranus so that astronomers can study their structure. In reality, the rings are as dark as black lava or charcoal.

Uranus - system with rings This false-color view of the rings of Uranus was made from images taken by Voyager 2 on Jan. 21, 1986, from a distance of 4.17 million kilometers (2.59 million miles). The image shows that the brightest, or epsilon, ring at top is neutral in color, with the fainter eight other rings showing color differences between them. Moving down, toward Uranus, we see the delta, gamma and eta rings in shades of blue and green; the beta and alpha rings in somewhat lighter tones; and then a final set of three, known simply as the 4, 5 and 6 rings, in faint off- white tones. This color information can be used to try to understand the nature and origin of the ring material.

Neptune’s rings (similar to Uranus – dusty ambient atmosphere)